1. Field of the Invention
The present invention relates to communications, and, in particular, to code division multiple access (CDMA) communications systems.
2. Description of the Related Art
FIG. 1a shows a representation of a telecommunications network comprising base stations 102, mobile switching center (MSC) 104, and relay node 106. Network 100 is designed to support communications to and from remote terminals that are located within the coverage area of base stations 102. For example, if the remote terminals are mobile/cellular telephones then the network supports telephone communications to and from mobile phone users located within the network.
In operation, each remote terminal transmits telecommunications signals to and/or receives telecommunications signals from (at least) one base station 102. Each base station 102 transmits signals received from the remote terminals within its coverage area to MSC 104. MSC 104 identifies the appropriate destinations for the signals received from its remote terminals and distributes those signals accordingly (e.g., to a base station 102 or to relay node 106). Relay node 106 may be connected via hard-wire or air-link to other relay nodes and/or other types of communications networks. Similarly, relay node 106 receives signals from other communications networks for distribution to MSC 104. MSC 104 transmits signals back to base stations 102 for broadcast and receipt by the appropriate remote terminals. In this way, the network of FIG. 1a supports telecommunications to and from remote terminals within the coverage area of base stations 102.
Base stations 102 are preferably distributed to provide seamless coverage. In other words, base stations 102 are located such that, at any location within the total coverage range of the network, a remote terminal will be able to communicate with (at least) one base station 102.
FIG. 1b shows a representation of an idealized (e.g., perfectly flat) communications network 100, having base stations 102 distributed in such a manner as to provide seamless coverage over the entire network range. The effective range of each base station 102 in network 100 is depicted as a circle in FIG. 1b and is referred to as a cell site 108. In reality, cell sites may be more accurately considered to be three-dimensional (e.g., spherical or semi-spherical) in shape. The union of all of the cell sites 108 forms the total coverage area for network 100.
In FIG. 1b, base stations 102 are distributed such that the cell sites of adjacent base stations overlap and there are no locations within the interior of the network that are not covered by at least one base station. As shown in FIG. 1b, some locations (i.e., those within the intersections of two adjacent cell sites) may be able to communicate with two different base stations, while other locations (i.e., those within the intersections of three adjacent cell sites) may be able to communicate with three different base stations.
FIG. 2 shows the coverage pattern for telecommunications network 100 of FIG. 1b. Each circle in FIG. 2 corresponds to the cell site 108 of a different base station 102 in network 100. In FIG. 2, each base station 102 transmits and receives signals in an omnidirectional pattern. That is, each base station 102 transmits its signals uniformly in all directions (i.e., 360 degrees when viewing each cell site as a circle).
One communications scheme for use in networks such as telecommunications network 100 of FIG. 1b is the IS-95 standard, which is based on code division multiple access (CDMA) modulation. According to the IS-95 standard for CDMA systems, each base station 102 of FIG. 1b is assigned a different pseudo noise (PN) offset (or, at least, adjacent base stations are assigned different PN offsets). In this way, each base station 102 can support up to 64 different code channels, with each code channel being assigned one of 64 different orthogonal Walsh code (i.e., CDMA) sequences.
Under the IS-95 standard, there are five different types of code channels that can be used to communicate between a base station 102 and each remote terminal: pilot, sync, paging, access, and traffic. The forward link (from base station to remote terminal) has the following four types of channels:
Pilot--transmitted at a high power level and providing a reference for decoding sync, paging, and forward link traffic channels; PA1 Sync--providing timing information to the remote terminal: PA1 Paging--providing cell site information to the remote terminal; and PA1 Traffic--providing power control data and voice data to the remote terminal. PA1 Access--providing a means for a remote terminal to initiate a call or respond to a page; and PA1 Traffic--providing a means for the remote terminal to send power control data and voice data to the base station.
The reverse link (from remote terminal to base station) has the following two types of channels:
Under the IS-95 standard, for each base station, the forward link has one pilot channel (typically assigned Walsh code sequence 0), one sync channel (typically assigned Walsh code sequence 32), and (in theory) up to 62 different paging and traffic channels (each assigned one of the 62 remaining Walsh code sequences). Similarly, the reverse link has one or more access channels and one traffic channel for every traffic channel in the forward link (i.e., up to 62). Instead of using Walsh codes for the reverse link, each traffic channel is identified by a distinct user long code sequence and each access channel is identified by a distinct access channel long code sequence.
In practice, however, the number of traffic channels (and therefore the number of remote terminals) that can be simultaneously supported by any one base station 102 of FIG. 1b is limited to much less than 62. Despite the mathematical orthogonality between channels that are assigned different Walsh code sequences, interference will still occur between those channels. This interference increases as more channels are assigned until the level of interference adversely affects the integrity of the communications. Depending upon the circumstances, this interference can limit the number of remote terminals capable of being supported at one time by a single base station to as low as about 10.
One conventional technique for increasing base station capacity (as well as coverage area) relies on sectorization. In sectorization, omni-directional cell sites are each divided into multiple sectors to achieve the desired capacity and coverage. Sectorization provides a way to divide the total number of users (One user per traffic channel) into smaller groups. Assume, for example, that all users are evenly distributed by location around a base station. A sectorized antenna system uses directional antennas to divide the cell site like slices of a pie.
FIG. 3 shows a representation of a sectorization scheme for telecommunications network 100 of FIG. 1b, in which each cell site is sectorized into three equal sectors 110. In other sectorization schemes, each cell site may be divided into a different number of sectors. In general, however, each sector within a given cell site is assigned a different PN offset. Because each sector is assigned its own PN offset, each sector has its own pilot channel. Thus, in FIG. 3, each cell site 108 transmits three different pilot channels, one for each sector 110. Since each sector of a given cell site has its own PN offset, each sector is capable of supporting 64 different code channels. As a result, the sectorization scheme of FIG. 3 theoretically triples the number of remote terminals that can be supported by a single base station.
For CDMA systems, like network 100 of FIG. 1b, however, the benefits of sectorization are limited. Some of these limitations relate to cell site capacity, pilot pollution, and hand-off processing.
As to cell site capacity, interference between code channels effectively limits the number of useable code channels. Ideally, the amount of interference in each sector is reduced since it is based on the subset of users in its geographic slice. In practice, however, the interference reduction is based primarily on the antenna directivity (or pattern), overlap of sector boundaries, and the uneven distribution of mobile terminals in the cell site.
As to pilot pollution, a conventional CDMA remote terminal can capture forward link energy from as many sources as the number of RAKE fingers in the remote terminal. For example, a three-finger RAKE receiver can capture energy from a maximum of three sectors or multipaths. This energy can come from multipath in a scattering environment, from multiple sectors in a single base station, and/or from multiple base stations. Once the remote terminal has assigned all of its RAKE fingers to the strongest paths, any additional energy received from other paths acts as interference. Therefore, if the remote terminal is receiving energy from excess sectors and/or reflected paths, the forward link performance can be degraded. Higher orders of sectorization can adversely affect forward link performance by increasing tile chances of interference from additional signal paths. For example, in the sectorization scheme of FIG. 3, there are locations (i.e., some of the intersections of three cell sites) that will receive up to 6 different pilot channels, not even counting any multipath. Since a three-finger RAKE receiver can receive only up to three pilot channels, the other three pilot channels (in addition to any multipath) will contribute to the level of interference.
As to hand-off processing, the process of conducting soft hand-offs between base stations and softer hand-offs between sectors of a base station is a complicated series of events involving the mobile switching center (MSC) and the remote terminal. Higher orders of sectorization will only increase that complexity.
The present invention addresses problems of using sectorization in a CDMA system. These problems relate one or more of cell site capacity, pilot pollution, and hand-off processing.
Further aspects and advantages of this invention will become apparent from the detailed description which follows.